1. Field of the Invention
The present invention relates to a printing apparatus which applies a printing agent on a printing medium from printing means with a plurality of printing elements arranged therein to form an image. In particular, the present invention relates to a method and a configuration for adjusting a printing position displacement of the printing element.
2. Description of the Related Art
A printing apparatus having a function of a printer, a copier or a facsimile, or a printing apparatus used as an output device of a composite electronic device including a computer or a word processor or a workstation prints an image on a printing medium such as a paper or a thin plastic sheet based on image information (including character information). Such printing apparatuses can be classified into an ink jet type printing apparatus, a wire dot type printing apparatus, a thermal type printing apparatus or a laser beam type printing apparatus according to the printing methods. Among the above, an ink jet type printing apparatus is the one that ejects ink from printing means (a printing head) to a printing medium for printing and has a number of excellent characteristics such as realizing high definition more easily, allowing high speed printing in excellent quietness and achieving a lower cost compared to the other printing methods. Therefore, the ink jet printing apparatuses are now generally used over a wide area from an office to personal use.
In general, each of the ink jet printing apparatuses is provided with a printing head in which a plurality of printing elements, each of which includes an ink ejection port and a liquid channel for supplying ink to the port, are integrated and arranged. Further, so as to correspond to color images, each of the ink jet printing apparatuses is equipped with such printing heads of a plurality of colors.
The ink jet printing apparatuses are generally classified into a serial type printing apparatus and a line type printing apparatus from the difference of the printing operations. In the serial type printing apparatus, main print scanning in which a printing head moves and scans a printing medium to form an image, and sub-scanning in which the printing medium is carried in a direction intersecting the main print scanning are intermittently repeated to form an image. On the other hand, in the line type printing apparatus, a printing head in which a number of printing elements in response to a printing width of a printing medium are arranged is fixedly disposed, and while printing by the printing head is carried out, the printing medium moves in a direction different from an arrangement direction of the printing element at a predetermined speed, and thus an image is formed.
The line type printing apparatus can print at a high speed but the size of the device itself is likely to be large. On the other hand, the serial type printing apparatus can correspond to printing mediums of various sizes with a small printing head, and by changing the number of printing scanning or a main scanning direction to the same image area, correspond to various printing speeds and image quality in response to the user's preference. Thus, in these years, the serial type ink jet printing apparatuses are widely used especially for personal use.
However, the serial type ink jet printing apparatus includes problems peculiar to itself. In the serial type ink jet printing apparatus, main print scanning in which the printing head which ejects ink moves and scans the printing medium and sub-scanning in which the printing medium is carried in a direction intersecting the main print scanning are intermittently repeated to form an image on the printing medium. When there is an intention to output an image at as high a speed as possible, bidirectional printing to perform the main print scanning described above bidirectionally is generally adopted. At this time, when a printing position displacement (hereinafter also referred to as a bidirectional registration displacement) is included in forward scanning and backward scanning of the main print scanning, a negative effect on the image as follows is identified in some cases.
FIGS. 1A to 1D are drawings for explaining a bidirectional registration displacement phenomenon and the negative effect. Each of FIG. 1A and FIG. 1B shows the case of printing a ruled line pattern. Here, a reference numeral 501 shown in a solid line denotes a ruled line printed in the forward scanning and a reference numeral 502 shown in a broken line denotes a ruled line printed in the backward scan. When the bidirectional registration is not deviated, the ruled line 501 printed in the forward pass and the ruled line 502 printed in the backward pass are printed on the same straight line to form a straight ruled line pattern as in FIG. 1B. As opposed to this, when the bidirectional registration is deviated, the ruled line 501 printed in the forward pass and the ruled line 502 printed in the backward pass are printed at positions separated from each other, which makes the ruled line pattern into cut pieces as in FIG. 1A.
Moreover, when multi-pass printing is adopted and a same image area of the printing medium is divided in the forward scanning and backward scanning for printing, another problem is generated. Each of FIG. 1C and FIG. 1D show the case where a uniform pattern is printed in the multi-pass printing. Here, a reference numeral 503 shows a dot printed in the forward scanning and a reference numeral 504 shows a dot printed in the backward scanning. When the bidirectional registration is not deviated, the dots 503 printed in the forward pass and the dots 504 printed in the backward pass maintain a complementary relationship each other and are dispersed in a preferable state as in FIG. 1D to be printed. As opposed to this, when the bidirectional registration is deviated, the complementary relationship between the dots 503 printed in the forward pass and the dots 504 printed in the backward pass becomes incomplete, and the printing is performed in the state in which dot density variations are deviated as in FIG. 1C. The image as in FIG. 1C is perceived visually as image granularity, which is a factor of the image deterioration.
In order to solve the problems of the bidirectional registration displacement as described above, a method and a configuration to adjust bidirectional registration in an ink jet printing apparatus which performs the bidirectional printing have been devised and implemented (for instance, refer to Japanese Patent Application Laid-Open No. 7-81190).
However, according to the results obtained through the diligent examination by the inventors, even if the method described in Japanese Patent Application Laid-Open No. 7-81190 is adopted to adjust the bidirectional registration, when a slight inclination is included in the printing head, it has been confirmed that not only are the problems not solved sufficiently but also there is a case where a new negative effect is generated at the same time.
FIG. 2 is a drawing for explaining a printing head which has inclination. Here, the case of printing a ruled line extended in the sub-scanning direction is shown. When the printing head is inclined, even if the bidirectional registration is adjusted, the ruled line printed in each of print scanning is inclined. The printing state in the case where the bidirectional registration is adjusted with the use of such a printing head will be explained as follows.
FIGS. 3A and 3B are drawings, each showing a printing state in the case where the multi-pass printing is carried out with the use of a printing head which is not inclined. In the example, a serial type ink jet printing apparatus which forms an image at the printing density of 1200 dpi (dot/inch) is used and it is possible to adjust the bidirectional registration by 1 pixel, in other words, by 1/1200 inch. In FIG. 3A, an area shown in the pattern of a reference numeral 601 denotes an image of 1 pixel width printed in the forward scanning and an area shown in the pattern of a reference numeral 602 denotes an image of 1 pixel width printed in the backward scanning. In the example, a multi-pass (two-pass) printing is adopted, and after one print scanning is performed by the forward scanning or backward scanning, the printing head is moved by half the printing width in the sub-scanning direction to the printing medium.
When an amount of displacement of the bidirectional registration is 0, there is no displacement between the image 601 printed in the forward scanning and the image 602 printed in the backward scan, and both of the images overlap on the same straight line. As the amount of displacement of the bidirectional registration is gradually increased, the image 601 printed in the forward scanning and the image 602 printed in the backward scanning are gradually separated from each other.
FIG. 3B is a drawing for explaining an extent of each of image quality items in the case where the amount of displacement of the bidirectional registration is gradually changed as shown in FIG. 3A. Here, as the image quality items, “banding” and “granularity” are listed. In the present specification, “granularity” shows a sense of roughness which is increased corresponding to an extent of uneven of dot density variations in a uniform pattern as explained in FIG. 1C. For instance, the granularity in FIG. 1C is inferior, compared to that in FIG. 1D. On the other hand, “banding” means a non-uniform state to the sub-scanning direction perceived in the case where a state of dot density variations is changed in the sub-scanning direction. The granularity and banding are based on the causes mentioned above and recognized as items deteriorating the image quality visually. The evaluation shown in FIG. 3B is a result obtained through visual recognition by the inventors.
In FIG. 3B, when the amount of displacement of the bidirectional registration is 0, dots printed in the forward scanning and dots printed in the backward scanning are complemented in a preferable state for each other as in FIG. 1D. Because of that, the uneven of dot density variations is not generated and both of the banding and granularity are not recognized. When the amount of displacement of the bidirectional registration is gradually increased, the complementary relationship between the dots printed in the forward scanning and the dots printed in the backward scanning becomes insufficient, and the extent becomes more remarkable as the amount of displacement is larger. As a result of that, as the amount of displacement of the bidirectional registration becomes larger, the granularity is more deteriorated. In the case of the example, however, even in a deteriorated state, there is no factor of fluctuation of the extent of granularity to the sub-scanning direction. Thus, banding is maintained in an excellent state regardless of the amount of displacement of the bidirectional, registration.
On the other hand, FIG. 4A and FIG. 4B are drawings each showing, as in FIG. 3A and FIG. 3B, a printing state when the multi-pass printing is performed with the use of the printing head which is inclined. Here, the state in which the displacement of approximately 1 pixel, that is, nearly 1/1200 inch is included between a leading end and a trailing end of the printing head in the sub-scanning direction is shown. As shown in FIG. 4A, when the amount of displacement of the bidirectional registration is 0, even if there is the inclination, an area printed in the forward scanning and an area printed in the backward scanning are overlapped with each other almost preferably in the main scanning direction. Therefore, a nearly excellent complementary relationship in dots printed in the forward pass and dots printed in the backward pass is realized and thus, granularity is not perceived. Also, the state of dot density variations as described above does not fluctuate to the sub-scanning direction and thus, banding is also maintained in an excellent state.
On the other hand, the state in the case where the bidirectional registration displacement is generated will be explained on the basis of the case where the amount of displacement is +2. Here, a reference numeral 701 shows an image area printed in a first print scanning, a reference numeral 702 shows an image area printed in a second print scanning and a reference numeral 703 is an image area printed in a third print scanning. The image areas 701 and 703 are printed in the forward scanning and the image area 702 is printed in the backward scanning. The bidirectional registration is deviated by 2 pixels between the forward scanning and the backward scanning, and thus only the image area 702 by the second print scanning is formed at the position separated from the image areas 701 and 703. However, in the case, the distance between the image area printed in the forward direction and the image area printed in the backward direction is different, depending on the areas on the printing media. That is, while the distance between the image area 701 and the image area 702 is relatively shorter in the area A, the distance between the image area 703 and the image area 702 is relatively longer in the area B. When a uniform pattern is printed in such a state, a state of complementarity of dots, in other words, a state of density variations is different between the area A and the area B. As a result of that, two kinds of areas thereof are repeated in the sub-scanning direction, which generates banding to be recognized.
FIG. 4B is a drawing for explaining the extent of banding and granularity when the amount of displacement of the bidirectional registration is gradually changed as in FIG. 4A. In the drawing, when the amount of displacement of the bidirectional registration is 0, the dots printed in the forward scanning and the dots printed in the backward scanning are nearly in the state of complementarity. Because of that, the uneven of dot density variations itself is not generated and both of banding and granularity are not recognized. When the amount of displacement of the bidirectional registration is gradually increased, the complementary relationship between the dots printed in the forward scanning and the dots printed in the backward scanning becomes insufficient, and the extent becomes more remarkable as the amount of displacement is larger. As a result of that, as the amount of displacement of the bidirectional registration becomes larger, the granularity is more deteriorated. Also, in the state in which the printing head is inclined as in the example, areas of each of which a state of density variations is different as in the area A and the area B explained in FIG. 4A are arranged alternately in the sub-scanning direction and thus, it is likely to deteriorate the extent of banding.
Such banding is a negative effect which is generated compositely from two factors of the bidirectional registration displacement and head inclination. The inventors, as a result of the diligent examination, have confirmed that even if there is only a little bidirectional registration displacement and head inclination respectively, a negative effect by the banding described above is noticeable earlier than the direct negative effects such as granularity and ruled line displacement. That is, referring to FIG. 4B again, when the amount of displacement of the bidirectional registration is −1, even if the granularity is not deteriorated so much, the banding is already deteriorated to the extent where it is recognized.
Further, what is noted is that whether the amount of displacement of the bidirectional registration is extended in a positive direction or a negative direction, the granularity is deteriorated by approximately the same extent, but as for the banding, a degree of deterioration thereof is different depending on the positive and negative directions. The inventors has focused on the point and confirmed how the state of banding as shown in FIG. 4B is changed when a degree of inclination of the printing head is further fluctuated.
FIG. 5 is a drawing showing a result of the examination mentioned above. Here, the result obtained when the inclination of the printing head is set to ±4 is shown as in FIG. 3B and FIG. 4B. As in the printing head explained in FIG. 4A, an inclination of the extent where an approximately 1 pixel displacement is generated between the leading end and the trailing end of the printing head is here referred to as “inclination 1”. And, the states in which the direction is the same as the above and the amount of displacement between the leading end and the trailing end is increased by 1 pixel are shown respectively as “inclination +2”, “inclination +3” and “inclination +4”. On the other hand, the states in which the direction of the inclination is reversed are shown as “inclination −1” to “inclination −4”.
As is apparent also in the drawing, even if the amount of displacement of the bidirectional registration is approximately ±1 pixel, when there is an inclination, a negative effect by banding is identified. For instance, when the inclination is ±1, it is determined as approximately “Δ” and when the amount of inclination is further increased, it is determined as approximately “x”.
Until now, for the correction of the bidirectional registration, a plurality of patterns changed in relation to the amount of displacement of the bidirectional registration have been printed simultaneously while the amount of displacement is changed step by step. Then, the plurality of pattern printed has been confirmed through visual inspection by the user or detection means such as a sensor to select a pattern of the least amount of displacement of the bidirectional registration. Furthermore, the print timing at which the selected pattern was printed have been set, by which the bidirectional registration have been generally adjusted. However, in the bidirectional registration adjustment pattern printed in such a method, the extent of the banding generated by the effect of the head inclination can not be identified.
On the other hand, in an ink jet printing apparatus allowing the high resolution image output these days, a bidirectional registration displacement of approximately ±1 pixel is generated suddenly or steadily because of various factors in some cases. Moreover in the adjustment of the bidirectional registration, in many cases, a displacement of approximately ±1 pixel is in a range where it is accepted as an error. Therefore, in the ink jet printing apparatus, it is strongly desired that even when the amount of displacement of the bidirectional registration is approximately ±1 pixel, the image is the one such that a large negative effect is not visually recognized so much.
However, as explained above, when the printing head is inclined, even when the bidirectional registration displacement is approximately ±1 pixel, the banding is easily identified, which as a result deteriorates the image quality. In particular, as visual characteristics of human beings, a band-shaped repetition as shown in banding rather than uniform roughness shown in granularity is felt to be uncomfortable in more cases. Also from such a reason, reducing the banding described above is a very important challenge.